Refactor UnmarshalText in order to parse hash-like UUID.

[daskol]

I refactor UnmarshalText in order to support yet another text representation of UUID lilke 6ba7b8109dad11d180b400c04fd430c8. The reason is that this format is quite frequent.

I tried to introduce BNF of what UmarshalText actually does. In fact it simplifies implementation and future support. On other hand current implementation led to performance degradation(see below before/after).

BEFORE

BenchmarkFromString-4               	10000000	       122 ns/op
BenchmarkFromStringUrn-4            	10000000	       124 ns/op
BenchmarkFromStringWithBrackets-4   	10000000	       123 ns/op
BenchmarkUnmarshalText-4            	20000000	        81.6 ns/op
PASS
ok  	github.com/daskol/go.uuid	25.786s

AFTER

BenchmarkFromString-4               	20000000	        91.7 ns/op
BenchmarkFromStringUrn-4            	 3000000	       493 ns/op
BenchmarkFromStringWithBrackets-4   	 5000000	       358 ns/op
BenchmarkUnmarshalText-4            	 2000000	       612 ns/op
PASS
ok  	github.com/daskol/go.uuid	27.822s

Eventually correct UnmarshalText should be based on FSA to avoid overhead during parsing.

[coveralls]

Coverage decreased (-1.9%) to 94.702% when pulling 6902e4b on daskol:parse-from-string into 5bf94b6 on satori:master.

[daskol]

It is wondered that simple heuristics improves perfomance significantly.

NOW

BenchmarkFromString-4               	20000000	        92.3 ns/op
BenchmarkFromStringUrn-4            	20000000	       102 ns/op
BenchmarkFromStringWithBrackets-4   	20000000	       106 ns/op
BenchmarkUnmarshalText-4            	10000000	       156 ns/op
PASS
ok  	github.com/daskol/go.uuid	28.302s

[coveralls]

Coverage decreased (-5.7%) to 90.82% when pulling 23c4681 on daskol:parse-from-string into 5bf94b6 on satori:master.

[coveralls]

Coverage decreased (-5.7%) to 90.82% when pulling 23c4681 on daskol:parse-from-string into 5bf94b6 on satori:master.

[daskol]

@satori Your resolution needed!

[satori]

@daskol I refactored decodeCanonical to remove extra memory allocations which caused performance degradation. That and few other style fixes were merged into 8be4f1c

Thanks for your contribution.